Retaining Wall Block Calculator
Works out blocks, caps, base gravel and drainage backfill for a segmental retaining wall. Enter the exposed height — the buried first course is added for you.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
blocks per course = wall length ÷ block face length courses = (exposed height + buried depth) ÷ block height blocks = courses × blocks per course × (1 + waste)
The only non-obvious term is the buried course. A segmental wall is held by its own weight and the soil in front of the bottom course, so roughly a tenth of the exposed height goes below grade — and those blocks still have to be bought.
What to watch out for
Three things matter more than the block count:
- Bury the first course. About a tenth of the exposed height, on compacted gravel. This is what stops the base sliding out.
- Drain behind it. 300 mm of clean stone with a perforated pipe at the bottom going somewhere. Water pressure behind a wall is what pushes walls over, and it builds up fast in clay.
- Over 1.2 m needs engineering. Taller walls, or any wall holding back a slope or a driveway, need geogrid reinforcement designed for the actual soil. Many places require a permit and a drawing.
Frequently Asked Questions
How many blocks for a retaining wall?
Divide the wall length by the block face length for the count per course, then divide the total height — exposed plus buried — by the block height for the number of courses. An 8 m wall 1 m high in 300 × 150 mm blocks needs about 8 courses of 27, so around 230 blocks.
How deep should a retaining wall base be?
Bury roughly a tenth of the exposed height, with at least one full course below grade, on 150 mm of compacted gravel. A 1 m wall therefore starts about 100–150 mm below the finished ground level.
Do I need drainage behind a retaining wall?
Yes. 300 mm of clean stone against the back of the wall and a perforated pipe at the base, leading somewhere it can discharge. Hydrostatic pressure is the usual reason retaining walls fail.
How tall can a retaining wall be without engineering?
Commonly about 1.2 m, but it depends entirely on where you are and what is behind it. Any wall supporting a slope, a structure or a driveway needs design regardless of height. Check local requirements before building.